(a) Field of the invention
The present invention relates to electrophotographic photoreceptors and coating solutions for production of charge transport layers.
(b) Description of Related Art
A conventional type of electrophotographic photoreceptors are "Se" photoreceptors produced by vacuum evaporating a selenium (Se) coating of about 50 .mu.m thick onto a conductive support, such as an aluminum support, which however suffer at least from their limited sensitivity only to lights of about 500 nm or less in wavelength.
Another conventional type of photoreceptors have, on a Se layer of about 50 .mu.m thick disposed on a conductive support, an additional selenium-tellurium (Se-Te) alloy layer. They can be made spectrally sensitive to longer wavelengths by increasing the Te content in the Se-Te alloy layer, but suffer from the serious disadvantage that they lose the ability of keeping charge on surfaces as the Te content increases, to be inapplicable to practical use.
Further, there are so-called composite-two-layer-type photoreceptors, which contain, on an aluminum support, an about 1 .mu.m thick charge generation layer which is a coating of chlorocyane blue or a squalilium acid derivative, and, on the charge generation layer, an about 10 to 20 .mu.m thick charge transport layer which is a coating of a mixture of polyvinylcarbazole having high insulating resistance or a pyrazoline derivative and polycarbonate resin. They, however, are not sensitive to lights of 700 nm or more.
As improvements of the composite-two-layer-type photoreceptors, there have recently been proposed various photoreceptors sensitive to lights of around 800 nm which range is the oscillation region of semiconductor lasers. Many of these composite-two-layer-type photoreceptors have an about 0.5 to 1 .mu.m thick charge generation layer containing a phthalocyanine pigment as a charge generation material and, thereon, a charge transport layer which is an about 10 to 20 .mu.m thick coating of a mixture comprising polyvinylcarbazole, a pyrazoline derivative or a hydrazone derivative and polycarbonate resin or polyester resin and having high insulating resistance.
Examples of known charge transport materials used in charge transport layers are hydrazone derivatives disclosed in Japanese Patent Application Examined Publication No. 55-42380 (1980), enamine derivatives disclosed in Japanese Patent Application Unexamined Publication No. 62-237458 (1987), benzidine derivatives disclosed in Japanese Patent Application Examined Publication No. 59-9049 (1984) and Japanese Patent Application Unexamined Publication Nos. 55-7940 (1980) and 61-295558 (1986), stilbene derivatives disclosed in Japanese Patent Application Unexamined Publication No. of 58-198043 (1983) and triphenylamine derivatives disclosed in Japanese Patent Application Examined Publication No. 58-32372 (1983) and Japanese Patent Application Unexamined Publication No. 61-132955 (1986).
Known benzidine derivatives include N,N,N',N'-tetraphenylbenzidine, N,N'-diphenyl-N,N'-bis(3-methylphenyl)benzidine, N,N,N',N'-tetrakis(4-methylphenyl)benzidine, N,N'-diphenyl-N,N'-bis(4-methoxyphenyl)benzidine and N,N,N',N'-tetrakis(4-methylphenyl)tolidine. These benzidine derivatives transport charge relatively efficiently, but have poor solubility in organic solvents and are easily oxidized. Due to the poor solubility, the benzidine derivatives sometimes make it difficult to prepare coating solutions for the production of charge transport layers, or are crystallized during coating. Even if the charge transport layers visually have a good appearance, the benzidine derivatives in the charge transport layers deposit as fine crystals, to deteriorate image quality.
To solve these problems, in Japanese Patent Application Unexamined Publication No. 5-6010 (1993) are proposed electrophotographic photoreceptors of high sensitivity and good image characteristics, which contain new fluorine-containing N,N,N',N'-tetraarylbenzidine derivatives having good solubility in organic solvents and excellent compatibility with binders, such as polycarbonate resins.
However, electrophotography, typically in laser beam printers, is being advanced in image quality and resolution, requiring electrophotographic photoreceptors having higher sensitivity, lower residual potential and better image quality.
Charge generation materials which have been used in combination with these charge transport materials include metal-free phthalocyanines and metallo-phthalocyanines, such as copper phthalocyanine, chloroaluminum phthalocyanine, chloroindium phthalocyanine, titanyl phthalocyanine and vanadyl phthalocyanine.
Phthalocyanines differ from each other in absorption spectrum and photoconductivity according not only to the kinds of central metals but also to the crystal structures thereof. There are some reports of selecting ones of specific crystal structures for electrophotographic photoreceptors from phthalocyanines containing the same central metal.
For example, there are various titanyl phthalocyanines of different crystal structures, which are reported to be largely differ in charging efficiency, dark decay ratio (herein, "dark decay ratio" means the ratio of a surface potential remaining after standing in the dark to an initial surface potential before the standing) and sensitivity depending on their crystal structures.
In Japanese Patent Application Unexamined Publication No. 59-49544 (1984), a titanyl phthalocyanine of a crystal structure which has a diffraction spectrum indicating intense peaks at Bragg angles (2.theta..+-.0.2.degree.) of 9.2.degree., 13.1.degree., 20.7.degree., 26.2.degree. and 27.1.degree. is described to be desirable, with an X-ray diffraction spectrum thereof shown therein. An electrophotographic photoreceptor produced by using this titanyl phthalocyanine as a charge generation material has a dark decay ratio (DDR) of 85% and a sensitivity (E.sub.1/2) of 0.57 lux.multidot.sec.
In Japanese Patent Application Unexamined Publication No. 59-166959 (1984) is disclosed a charge generation layer produced by allowing a deposition layer of titanyl phthalocyanine to stand in a saturated vapor of tetrahydrofuran to change its crystal structure. Its X-ray diffraction spectrum shows a decreased number of widened peaks and indicates intense diffraction peaks at Bragg angles (2.theta..+-.0.2.degree.) of 7.5.degree., 12.6.degree., 13.0.degree., 25.4.degree., 26.2.degree. and 28.6.degree.. An electrophotographic photoreceptor produced by using the titanyl phthalocyanine of the changed crystal structure as a charge generation material has a dark decay ratio (DDR) of 86% and a sensitivity (E.sub.1/2) of 0.7 lux.multidot.sec.
Japanese Patent Application Unexamined Publication No. 2-198452 (1990) discloses that a titanyl phthalocyanine having such a crystal structure as to give a major diffraction peak at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree. is produced by heating a titanyl phthalocyanine in a mixture of water and o-dichlorobenzene at 60.degree. C. for 1 hour with stirring and has a high sensitivity (1.7 mJ/m.sup.2).
Japanese Patent Application Unexamined Publication No. 2-256059 (1990) discloses that a titanyl phthalocyanine of such a crystal structure as to give a major diffraction peak at a Bragg angle (2.theta..+-.0.2.degree.) of 27.3.degree. is produced by stirring a titanyl phthalocyanine in 1,2-dichloroethane at room temperature and has a high sensitivity (0.62 lux.multidot.sec).
Japanese Patent Application Unexamined Publication No. 62-194257 (1987) proposes to use mixtures of two or more kinds of phthalocyanines, such as a mixture of titanyl phthalocyanine and a metal-free phthalocyanine, as charge generation materials.
In Japanese Patent Application Unexamined Publication No. 6-175382 (1994) is proposed to produce a novel phthalocyanine composition which has a CuK.alpha.-X-ray diffraction spectrum indicating major peaks at Bragg angles (2.theta..+-.0.2.degree.) of 7.5.degree., 22.5.degree., 24.3.degree., 25.3.degree. and 28.6.degree., by precipitating a phthalocyanine mixture containing a titanyl phthalocyanine and a halogenometal phthalocyanine having a trivalent central metal in water using an acid-pasting method, and then treating the precipitate with an organic solvent.
In Japanese Patent Application Unexamined Publication No. 8-41373 (1996) is proposed to produce a novel phthalocyanine composition which has a CuK.alpha.-X-ray diffraction spectrum indicating major peaks at Bragg angles (2.theta..+-.0.2.degree.) of 9.3.degree., 13.1.degree., 15.0.degree. and 26.2.degree., by precipitating a phthalocyanine mixture containing a titanyl phthalocyanine and a halogenometal phthalocyanine having a trivalent central metal in water using an acid-pasting method, and then treating the precipitate with an organic solvent.
These phthalocyanine compositions transformed in crystal structures are useful as charge generation materials of good properties, but are not satisfactory for recent electrophotography, typically in laser beam printers, which is advanced in image quality and resolution and in requirement for electrophotographic photoreceptors having higher sensitivity, lower residual potential and better image quality.
In Japanese Patent Application Unexamined Publication No. 6-271786 (1994) is proposed a phthalocyanine composition and a method of production thereof, which has a CuK.alpha.-X-ray diffraction spectrum indicating major peaks at Bragg angles (2.theta..+-.0.2.degree.) of 7.5.degree., 24.2.degree. and 27.3.degree. and are more sensitive.
As described above, phthalocyanines largely differ in electrophotographic properties depending on their crystal structures, which are therefore important factors influencing the performance of electrophotographic photoreceptors. Particularly, phthalocyanine compositions provide charge generation materials which exhibit excellent properties because of their extremely high sensitivity.
However, electrophotography, typically in laser beam printers, is advanced in image quality and resolution, requiring electrophotographic photoreceptors having higher sensitivity, lower residual potential and better image quality.
It is also known that dark decay ratio, sensitivity and residual potential remarkably change depending on the combinations of charge generation materials and charge transport materials, and investigation into desirable combinations of these materials is required to obtain electrophotographic photoreceptors well-balanced in these properties.